1. Field of the Invention
This invention relates to an apparatus for encoding/decoding a composite color video signal and more particularly, to a multidimensional filter for the separation of the luminance and chrominance components of NTSC, PAL and ATV color television signals. The present invention also relates to a method for designing several classes of diamond shaped filter.
2. Description of the Prior Art
During the past few years, there has been an increasing use of vertical delay line comb filters for improving the horizontal separation of the luminance and chrominance composents in a composite TV signal. Several separable filters respectively in the horizontal and vertical domains have been implemented with analog or digital techniques as disclosed in U.S. Pat. Nos. 4,345,268; 4,500,912 and 4,524,400. However, this class of filters yield a resolution loss of diagonal high frequency luminance information. Therefore, some adaptation and/or compensation techniques were suggested such as disclosed in U.S. Pat. Nos. 4,040,084 and 4,240,105. However adaptation artifacts can be introduced in the viewed image.
Recently there was introduced the diamond shaped spectrum of the chrominance components in an NTSC encoded signal, and U.S. Pat. No. 4,829,367 teaches the use of non-separable horizontal vertical diamond shaped filters for NTSC encoding and decoding. These non-separable filters are generally complex and the luminance bandstop filter performance is not ideal along the horizontal and vertical frequency domain axes.
In the vertical temporal domain, U.S. Pat. No. 4,683,490 describes the use of diamond shaped filters implements with odd and even field delays and suitable lowpass and highpass filter coefficients. However, the proposed twelve field filter yields a limit of 15 dB for component separation along the zero axes. The inventors have also suggested the use of frame sampled filters which are simply rectangular shaped bandpass or bandstop filters in the vertical temporal frequency domain.
In the case of encoding/decoding a PAL video signal, the situation is quite similar. In an article by J.O. Drewery, entitled "The Filtering of Luminance and Chrominance Signals to Avoid Cross-Colour in a PAL Colour System", BBC Engineering, 8-39, September 1976, there is proposed some (separable) rectangular or (non-separable) circular shaped filters in the spatial frequency domain. The choice of circular shaped region remains intuitive and yields a relatively simple filter calculation. In the vertical-temporal domain. Drewery and C.K.P. Clarke, in an article entitled "PAL Decoding: Multidimensional Filter Design for Chrominance-Luminance Separation", BBC Research Department Report no BBC-RD 1988/11, have suggested also the use of odd and even field delays for diamond shaped filter implementation. The filter performance is similar to that of filter performance of U.S. Pat. No. 4,683,490, in the case of NTSC signal.
Finally, it is noted that all existing proposed filters have the diamond shaped region in either the horizontal-vertical or vertical-temporal domain.